3-3 worked for me also. I thought I had killed a bunch throughout the campaign but I always softened them up first to make sure one grenade would do the job. After reading the comment about killing them purely with grenades I tried that and got it in two short runs.
I did do a few kills with the Magnetar (sp?) method also mentioned so that may have contributed but it unlocked right at the start of 3-3 after killing the 2nd watcher. Screwy achievement but at least it unlocked.